What does a pKa value tell you?

In simple terms, pKa is a number that shows how weak or strong an acid is. A strong acid will have a pKa of less than zero. More precisely – pKa is the negative log base ten of the Ka value (acid dissociation constant). It measures the strength of an acid — how tightly a proton is held by a Bronsted acid.

How does pKa affect drug solubility?

The higher the pKa, the stronger the acid ( the more dissociated into protons). Generally speaking the pH of extracellular fluid is always going to be within some decimal fractions of 7.4, and so drugs with a pKa under 7 (i.e. weak acids) will usually be water-soluble.

How does pKa affect drug distribution?

So, in this case pH = pKa. Hence, when pH is equal to pKa, the drug is ionized halfly. Ionization of drug effects not only the rate at which the drug permeate membrane but also steady state distribution of drug between the body compartments, if pH difference is present between them.

What is the pKa rule?

But for practical purposes, a good rule of thumb is about 10 pKa units. That is to say, if the difference in pKas between an acid and a base (actually, the conjugate acid of the base) is about 10 pKa units or less, it is useful to consider their acid-base reaction to be in equilibrium.

What are the importance of pKa in pharmacy?

A knowledge of the ionization constants (pKa) of compounds is important for much of the work carried out in the drug discovery process. These constants can have a profound affect on the physicochemical properties of a compound and, in rational drug discovery, are essential for the optimization of ADME characteristics.

How does pH affect drug distribution?

Urine pH is a great influence on whether a drug is excreted quickly or slowly and in some clinical situations is manipulated to control the excretion of certain drugs from the body. Most drugs are either weak acids or weak bases. In alkaline urine, acidic drugs are more readily ionised.